In the 20th cent., Arab leaders have attempted to form an Arab nation, which would unite the whole Arabic-speaking world from Morocco on the west, across the Middle East, to the borders of Iran and Turkey. Since 1945 most of the Arab nations have combined to form the Arab League, its purpose being to consider matters of common interest, such as policy regarding Israel and colonialism. With 22 member states in the Arab League by the mid-1990s, attempts to forge a unity among the Arabs have continued. Perhaps the most significant economic factor for the Arabs has been the discovery and development of the petroleum industry; two thirds of the world's oil reserves are thought to be in the Middle East. Since World War II a continual problem for the Arab states has been their relations with the Jewish state of Israel, created out of former Arab territory; hostility between them has resulted in four Arab-Israeli wars.